Cascade phosphor screen



July 19, 1949. V ,F H, NICOLL 2,476,619

CASCADE PHOSPHOR SCREEN Filed Aug. 23, 1946 IN V EN TOR.

Patented July V19, 1949 Radio Corporation of America,

of Delaware a' corporati'om application' August 28, 19461, serizr No. 8921451 7 Claims.

'This inventionl relates. to' phosphor screens in cathodei ray :tubes and is particularly useful in radar tubesz-though. not. limited thereto.

Iir. certain: types. of cathod'e. ray tubes. the= phosphsor screen-islcascade excited` `In such tubes the screenconsists: of, say. two layers, one adapted to emit. light under-bomb'ardment by: the. beam, md: another' adapted toremit light: under excitatimrv by the light emitted by the` first layer. In certain uses the:l light emittedby' the layer under lbeam' bombardmenti's desirable only for excitationi of' the. second? layer and' its transmission through the' second; layer to the observer along with the light vemitted by the second layer is dbtrimental.. One: example of this is. in radar tubes; i

There are certain phosphors that` have much longer persisteneell when.`excited by light of a particular wavelength, for example blue light, than when excited* byv bombardment of electrons. .In radar tubes a. phosphor of-relatively-long persistence is desirable and therefore phosphor screens for such tubes have been made of two layers, one to-beexcited by the'cathod'e ray beam of theztube for productionof blue light, for example lo-frelatively 'short' .pensistence a'nd': the other fon excitation by the emitted blue light to produce, say; yellow lightl.

In cascade screens for radar tubes as referred to the blue light produced" by'the one phosphor is not a-ll absorbed by the'other phosphor and this produces an undesiredf bluefiash in the trace. This fiash obscures the yellow afterfiow and in addition spoils the dark adaptation of the eye. To eliminate this blue fiash, filters have been placed in front of both screens that will pass the yellow light and absorb the blue light but this is ineflicient.

It is an object of this invention to provide a cascade phosphor screen in which that portion of the exciting light emitted by one layer, and passed through the layer excited thereby, is refiected by a selective refiector back thereinto, while the light emitted by the latter layer is not refiected but is transmitted.

Another object of the invention is to provide a two layer cascade phosphor screen with a selective refiector to reflect the light emitted by one phosphor layer and to transmit the light emitted .by the other phosphor layer.

Still another object of this invention is to provide means for refiecting the blue light fiash of cascade screens back into the phosphor to produce additional excitation thereof.

. Another object of the invention is to provide 2 a;v cascade screen: with a. refiector for bluev light that will efiiciently transmit yellow light.

Other objects of theinvention will appear' in the following description, reference being had to the drawing, in which: i

Figi l' illustratesf a phosphor screen embodying the invention; y

Pig. 2 isa secti'onofa cathode ray tube embody'- in'g' the screen and' reflect'or.

Referring to Fig. 1 of the drawing, on the inner side of the end of the evacuated glass envelope off 'the tube'is depositedaselecti've refi'ector 2 that p'asses yellow light and reflects blue light, for example. There are various ways of' making sucha refiector' but a' suitable' one can be made of a p'lural'ity of very' thin successive layers of 'materials of different indexes of refraction as discliosed in the article by G'. L. Dimmick in the Journal of the' Society of Motion Picture Engineers, vol. XXXVIII, pages 36 to 44', January, 1942: Forv example a six layer film having alternate layers' ofl zinc sulfide and; thorium oxi-fluoride may be used. Sucha film refiects blue and transmits yellow light if each layer is one-fourth of the wavel'ength of blue light and the first layer on the glass is thorium oxi-fiuoridel and the last iS zinc sulfi'de.. Thorium oxi-fiuoride-has a refi'act'ive index of about. 1.48. while zinc sulfide has an index of about 2:3. Thisl reflector can be made to transmit about of the yellow light 'andref1'ect'85'7bl of the blue light.

The phosphor 3 is placed next to refiector 2 by any known method. It may consist essentially of zinc and cadmium sulfides, in about the ratio of 6 to 1, respective'ly, With copper activator. Onto layer 3 is laid down, by any method, the phosphor layer 4, emitting blue light under excitation by the beam B. This may consist of zinc sulilde with silver activator. Other phosphors may be used to produce light of the desired colors.

In operation, the cathode ray beam seans the cascade screen in known ways and produces blue light by impact of the beam electrons on the second layer 4. This blue light passes into the first layer 3 and causes it to emit yellow light. In present types of radar tubes that I have tested, about 30% of the blue light passes through the layer 3 and is entirely wasted and in addition reduces the visual efl'ect of the yellow light. In my improvement, this unused blue light is refiected by selective reflector layer 2 back into the layer 3 and causes it to emit more yellow light, thus, in effect, producing a regenerating action 3 in addition to eliminating the undesirable blue flash.

While the refiector slightly cuts down the desired yellow light, it more than compensates for this by the increased yellow light produced by the refiected blue light. The small amount of blue light passing the refiector is not objectionable.

Various means may be provided for utilizing the invention, but I have shown by way of example in Fig. 2 a cathode ray tube of common form having a screen embodying the invention. In this figure the gun G producing the beam B may haveV the usual first anode 5, grid 6 and cathode 1, the heater not being shown. The beam B projected through the apertures in the grid and first anode is scanned over the target by the defiecting coil unit 8, though electrostatic defiection may be used if desired. Ether magnetic or electrostatic focusing means may be used but for simplification the focusing means is not illustrated, being well known in the art. A wall coating or equivalent anode 9 may be used.

i Reference is made to my copending application, now U. S. Patent 2,439,181, April 16, 1948, which discloses an improvement of this invention.

While certain specific embodiments have been illustrated and described, it will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

I claim:

1. In luminescing screens, a phosphor adapted to em'it light of one color, means for projecting onto one side of said phosphor light of another color adapted to excite the phosphor to fiuorescence, and a sele'ctive reflector on the other side of the phosphor adapted to transmit the major part of the first mentioned light and to refiect the major part of the second mentioned light passing therethrough.

2. A composite fluorescent screen comprising a layer of phosphor material adapted to emit visible light when excited by shorter wavelength radiation, and a selective refiector adjoining said layer and adapted to reflect said shorter wave radiation and to transmit the visible light emitted by said layer.

3. A light transparent foundation, a thin selective refiector layer on said foundation adapted to reflect light predominantly of a wavelength less than 4000 and to transmit nght predominanny of longer wavelength, and a phosphor layer on first phosphor layer which generates short wavelength light upon excitation, a second phosphor 'layer exposed to and excited by the light from said first layer to generate light of longer wavelength, and a third layer underlying said second layer and adapted to reflect the short wave light generated by said first layer and to transmit the longer wavelength generated by said second layer.

6. In tubes having cascade screens, a phosphor layer adapted to produce blue light under impact of the beam electrons, another layer adjacent the first adapted to produce yellow light under excitation of said blue light and a refiector lying against the last mentioned phosphor layer adapted to transmit yellow light and reflect blue light.

7.In tubes having cascade screens, a phosphor layer adapted to produce light of one color of relatively short persistence under electron excitation, a; second phosphor layer adjacent the first phosphor layer adapted to produce light of another color of long persistence upon excitation by the first mentioned light and a reflector adjacent the last mentioned layer to transmit the light emitted by the second mentioned layer and refiect the light emitted by the first mentioned layer.

FREDERICK H. NICOLL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,124,225 Batchelor July 19, 1938 2,171,970 Brett Sept. 5, 1939 2243,09? Henderson May 27, 1941 FOREIGN PATENTS Number Country Date I 474,907 Great Britain May 9, 1937 

